Structure and magnetic ordering of a ferrimagnetic helix formed by manganese(II) and a nitronyl nitroxide radical

A chain compound of formula Mn(hfac)2NITPhOMe, where hfac = hexafluoroacetylacetonato and NITPhOMe = 2-(4-methoxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide, was synthesized, and the X-ray analysis showed that it crystallizes in the trigonal acentric P3(1) space group with a = 11.131 (3) angstrom, c = 20.846 (9) angstrom, and Z = 3. The structure consists of helices in which the Mn(hfac)2 moieties are bridged by the radicals coordinated through the two equivalent oxygen atoms. The magnetic susceptibility in the form chi-T vs T shows a marked increase on lowering the temperature, and a numerical fitting indicated a strong antiferromagnetic intrachain interaction (J = 344 cm-1). The uncompensation of the S = 5/2 spin of manganese(II) and S = 1/2 spin of the radical originates high resultant magnetic moments, and at ca. 4.8 K the system order three-dimensionally as confirmed by AC susceptibility measurements in nearly zero field. Magnetization measurements indicate that the system orders ferrimagnetically with S = 2, as expected for antiparallel alignment of S = 5/2 and S = 1/2. Single-crystal EPR spectra at variable temperatures revealed a pure xy anisotropy with the easy plane perpendicular to the helix axis c. The calculation of the dipolar anisotropy performed within the point dipole approximation reproduces the observed behavior. The transition temperature and the dipolar energies are compared with those of two other Mn(hfac)2NITR chains, and magneto-structural correlations that justify the experimental trend in critical temperature are presented. The acentric space group allows nonlinear optical activity, and preliminary measurements show a sizeable SHG activity at 1064 nm, which must essentially be associated with the radical.